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71.
Prabhakar R. Bandaru 《Journal of Materials Science》2007,42(5):1809-1818
A review on the syntheses and electrical characterization of Y-shaped multi-walled carbon nanotube morphologies is presented.
Modified thermal CVD processes, using Ti precursors, are used to grow Y-junctions of different geometries and distribution
of catalyst particles. It has been established that novel electrical switching behavior is feasible, where any one of the
three branches of the Y-junction can be used for modulating the electrical current flow through the other two branches. Current
blocking behavior, leading to perfect rectification, is seen which could be related to the interplay of the carrier lifetime
and the transit time. The overall goal is to investigate the possibility of obtaining novel functionality at the nanoscale,
which can lead to new device paradigms. 相似文献
72.
The average positron lifetime has been determined non-destructively and in-situ during fully symmetric push-pull fatigue experiments
in the bulk material of the aluminium alloys 2024 T3 and 7075 T6 using a servo-hydraulic fatigue testing system equipped with
a mobile positron beam produced by a 72Se/72As generator (initial activity of ≈ 0.9 MBq; average positron penetration depth ≥1 mm). Contrary to earlier investigations
on stainless steel using the same experimental approach, no variation of the average positron lifetime could be observed during
fatigue and neither early nor late stages of fatigue damage could be revealed. It is concluded that fatigue induced changes
of the defect spectra in technologically relevant aluminium alloys are masked for the present method by saturation trapping
in precipitates. A sufficiently high increase of the dislocation density and the creation of vacancy clusters must be confined
to the vicinity of propagating fatigue cracks or the fatal fatigue crack. Therefore the zone with sufficient detectable fatigue
damage has not enough statistical weight to modify the average positron lifetime of the aluminium alloy bulk material. 相似文献
73.
Dielectric relaxation behaviour of (1 − x)PMN − xPZ, for x = 0.10, 0.30 and 0.40 have been studied. The nature of relaxational behaviour was found to change with PZ concentration.
A crossover from a static freezing to critical slowing down like behaviour is observed with increase in Zr4+ concentration. We have used Glazounov and Tangastev criterion to distinguish freezing and critical slowing down like behaviour. 相似文献
74.
Daniel X. Gouveia Odair P. Ferreira Antonio G. Souza Filho M. G. da Silva J. A. C. de Paiva Oswaldo L. Alves Josué Mendes Filho 《Journal of Materials Science》2007,42(2):534-538
The thermal decomposition properties of Mg–Fe hydrotalcites were studied through in situ 57Fe Mössbauer spectroscopy and in situ X-ray diffraction. Abrupt changes in the quadrupolar splitting measured in the Mössbauer spectra revealed a phase transition from the starting lamellar structure to a new crystalline arrangement. By analyzing the Mössbauer parameters we show that the material is highly disordered in the 300–400 °C temperature range. This hypothesis is confirmed by the X-ray results whose diffractograms indicated the collapse of the lamellar structure and the formation of a solid solution. 相似文献
75.
Modelling of effects of ultrastructural morphology on the hygroelastic properties of wood fibres 总被引:1,自引:0,他引:1
Wood fibres constitute the structural framework of e.g. wood, paper, board and composites, where stiffness and dimensional
stability are of importance. An analytical modelling approach has been used for prediction of hygroelastic response, and assessment
of the stresses in thick-walled cylinder models of wood fibres. A wood fibre was idealised as a multilayered hollow cylinder
made of orthotropic material with helical orientation. The hygroelastic response of the layered assembly due to axisymmetric
loading and moisture content changes was obtained by solving the corresponding boundary value problem of elasticity. A simple
solution scheme based on the state space approach and the transfer matrix method was employed. This was combined with an analytical
ultrastructural homogenisation method, used to link hygroelastic properties of constituent wood polymers to properties of
each layer. Predicted hygroelastic response captured experimentally measured behaviour. Fibres that were constrained not to
twist showed a stiffer response than fibres allowed twisting under uniaxial loading. It was also shown that the ultrastructure,
i.e. the microfibril angle, will control the hygroexpansion in the same way as it controls the compliance of the cell wall.
Qualitative failure trends comparable with experimental observations could be established with stress analysis and a simple
plane-stress failure criterion. 相似文献
76.
Tanu Sharma Sanjeev Aggarwal Shyam Kumar V. K. Mittal P. C. Kalsi V. K. Manchanda 《Journal of Materials Science》2007,42(4):1127-1130
The UV-Visible absorption spectra of virgin and gamma irradiated (20–800 kGy) CR-39 polymer have been deduced by using Shimadzu
Double beam Double Monochromator UV-Visible Spectrophotometer (UV-2550). The existence of the peaks, their shifting and broadening
as a result of gamma irradiation has been discussed. Finally the indirect and direct band gap in virgin and gamma irradiated
CR-39 has been determined. The values of indirect band gap have been found to be lower than the corresponding values of direct
band gap. 相似文献
77.
Igor Studart Medeiros Erika R. M. Andreeta Antonio C. Hernandes 《Journal of Materials Science》2007,42(11):3874-3877
78.
Dielectric and thermal characteristics of gel grown single crystals of ytterbium tartrate trihydrate
Dielectric and thermal characteristics of gel grown single crystals of ytterbium tartrate trihydrate have been carried out.
The dielectric constant has been measured as a function of frequency in the range 2 kHz–1 MHz and temperature range 30–300 °C.
The dielectric constant increases with temperature, attains a peak near 215 °C, and then decreases as the temperature exceeds
215 °C. The dielectric anomaly at 215 °C is suggested to be due to phase transition brought about in the material. The dielectric
behaviour of the material is correlated with the results on thermal analysis. Thermogravimetric and differential thermal analysis
have been used to study the thermal characteristics of the material. The experimental results show that the material is thermally
stable up to 200 °C. The decomposition process occurs in two stages until ytterbium oxide is formed at 700 °C. The non-isothermal
kinetic parameters e.g., activation energy and the frequency factor have been evaluated for each stage of thermal decomposition
by using the integral method, applying the Coats–Redfern approximation. 相似文献
79.
Christian P. Romero Jonathan I. Avila Edgardo Cisternas Guerau B. Cabrera Alejandro L. Cabrera Kristiaan Temst Margriet J. Van Bael 《Journal of Materials Science》2007,42(18):7667-7672
Hydrogen desorption curves were obtained from a sample composed of a square arrangement of Co dots with average diameter of
4.4 μm, separated by a distance of 11.6 μm. A macroscopic sample of Co dots grown on a 2.5 × 2.5 cm Si substrate was made
by standard lithographic techniques and used in these experiments. Thermal programmed desorption (TPD) was performed under
ultra-high vacuum conditions. Hydrogen TPD curves were obtained from a 1 × 1 cm Co dots samples displaying a maximum of intensity
at 425 K. Hydrogen TPD curve was also obtained from 1 cm× 1 cm samples of Co films and Co foils for comparison. The hydrogen
TPD curves have decreasing intensity from the Co foils to the Co dots and finally to the Co films. This indicates that there
are more sites for hydrogen adsorption on the Co dots than in the Co films. This is a surprising result because there is approximately
8.7 times less Co atoms exposed in the Co dots that in the Co film sample. A desorption energy of 27 kcal/mol was obtained
for the Co dots suggesting that hydrogen is adsorbed on an hcp hollow site of the Co dot crystalline structure. 相似文献
80.
Alexander Katsman Shalom Cohen Menachem Bamberger 《Journal of Materials Science》2007,42(16):6996-7003
This work deals with the development of Mg-based alloys with enhanced properties at elevated temperatures. This is achieved
by precipitation of binary phases such as MgZn2 and Mg2Sn during the aging of these alloys. The aim of the present work is to develop and calibrate a model for precipitation hardening
in Mg-based alloys, as different types of precipitates form simultaneously. The modified Langer-Schwartz approach, while taking
into account nucleation, growth and coarsening of the new phase precipitations, was used for the analysis of precipitates’
evolution and precipitation hardening during aging of Mg-based alloys. Two strengthening mechanisms associated with particle-dislocation
interaction (shearing and bypassing) were considered to be operating simultaneously due to particle size-distribution. Parameters
of the model, R
N
i and k
σ i
, were found by fitting of calculated densities and average sizes of precipitates with ones estimated from experiments. The
effective diffusion coefficients of phase formation processes, which determine the strengthening kinetics, were estimated
from the hardness maximum positions on the aging curves. 相似文献